I've seen the ads and they did them on This Morning.
The end result it fab but I can easily see how it takes three hours as apparently (is this what they did with you?) they tape both your bottom eyelashes and one layer of the top ones out of the way, and each lash is applied in singles.
I don't see how they can last 8 weeks as eyelashes 'refresh' (can't think of the right word!) themselves more regularly than this, but the ads I've seen from the states say they can last this long if you have almost like an infill appointment in the middle.
Personally I love the ones you get in little clumps, look just as fab if you do them right but not nearly so faffy, and they've always lasted at least a week on me. That way you can charge less money and do them more often, as I think the Jinny Lash rrp is quite a lot!